Aging effects at room temperature and process parameters on 3D-printed poly (lactic acid) (PLA) tensile properties

Poly(lactic acid) (PLA) is a common material for fused deposition modeling (FDM), thanks to its affordability and biodegradability. However, modeling the behavior of 3D-printed PLA parts asks for an accurate material characterization. This paper is dedicated to highlight process and specimen structure parameters that should be considered in addition to the standards commonly used in the community. Two test standards for polymer tensile properties, ISO 527 and ASTM D638, are compared. These test standards have similar tensile results. However, the rupture location would suggest that they are not perfectly adapted to 3D printing, thus showing the difficulty in determining mechanical properties of 3D printed polymers. The main objective of this study is to identify which criterion, besides from printing parameters, influences the properties of 3D-printed PLA. First, different morphologies (presence of walls, top/bottom layers, overall thickness) of specimens are in tension. Tensile tests do not reveal significant difference in properties between the different specimens' morphology. However, microscope observation of their rupture section showed presence of cracks in specimens printed with contours caused by poor adhesion between contours and infill. Then impact of room temperature aging is studied by carrying out a mechanical, chemical, and thermal analysis of 3D-printed PLA specimens stored in the dark at room temperature. The ductility of the specimen decreases with time and stabilizes 5 days after printing due to room temperature aging. This behavior may be caused by physical aging, hypothesis that has been validated by differential scanning calorimetry tests.